U.S. patent number 5,514,996 [Application Number 08/160,966] was granted by the patent office on 1996-05-07 for photo-coupler apparatus.
This patent grant is currently assigned to Kabushiki Kaisha Toshiba. Invention is credited to Yoshiaki Aizawa.
United States Patent |
5,514,996 |
Aizawa |
May 7, 1996 |
Photo-coupler apparatus
Abstract
A photo-coupler apparatus has a light emitting element in the
primary side. The secondary side of this apparatus is comprised of
a photoelectromotive diode array, a light sensitive impedance
element series-connected to said array, a drive transistor, and at
least one output MOSFET connected to the output terminals of this
apparatus. The light sensitive impedance element comes into a large
impedance state when an optical signal from the light emitting
element is weak. In this case, the light sensitive impedance
element generates a sufficient voltage to activate the drive
transistor, in spite of the photocurrent being small. This results
in an improvement of the dynamic sensitivity of this apparatus.
When said optical signal is strong, the impedance element comes
into a small impedance state, thus providing the MOSFET with a
sufficient photo-current. This results in the shortening of
switching times of the output MOSFET.
Inventors: |
Aizawa; Yoshiaki (Kanagawa,
JP) |
Assignee: |
Kabushiki Kaisha Toshiba
(Kanagawa, JP)
|
Family
ID: |
18176246 |
Appl.
No.: |
08/160,966 |
Filed: |
December 3, 1993 |
Foreign Application Priority Data
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Dec 4, 1992 [JP] |
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4-325385 |
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Current U.S.
Class: |
327/514; 250/551;
257/E31.104 |
Current CPC
Class: |
H01L
31/161 (20130101); H04B 10/801 (20130101) |
Current International
Class: |
H01L
31/16 (20060101); H04B 10/00 (20060101); H01L
031/00 () |
Field of
Search: |
;307/311 ;250/551
;327/514,515,427,434 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0041317 |
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Feb 1989 |
|
JP |
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244213 |
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Oct 1991 |
|
JP |
|
8300746 |
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Mar 1983 |
|
WO |
|
Primary Examiner: Tran; Toan
Attorney, Agent or Firm: Finnegan, Henderson, Farabow,
Garrett & Dunner
Claims
What is claimed is:
1. A photo-coupler apparatus comprising:
a light emitting element for emitting light by an input signal;
a photoelectromotive diode array for generating photoelectromotive
force by receiving an optical signal from said light emitting
element;
a light sensitive impedance element series-connected to said
photoelectromotive diode array, said light sensitive impedance
element changing its impedance value according to the intensity of
said optical signal from said light emitting element, wherein said
light sensitive impedance element comes into a small impedance
state when said optical signal from said light emitting element is
strong and it comes into a large impedance state when said optical
signal from said light emitting element is weak;
at least one output MOSFET whose gate and source are connected to
respective ends of the series-connected circuit comprised of said
photoelectromotive diode array and said light sensitive impedance
element; and
a normally-ON drive transistor whose control electrode is connected
to the node of said photoelectromotive diode array and said light
sensitive impedance element, and whose one pair of conducting
electrodes are connected to the gate and the source of said output
MOSFET respectively.
2. A photo-coupler apparatus comprising:
a light emitting element for emitting light by an input signal;
a photoelectromotive diode array for generating photoelectromotive
force by receiving an optical signal from said light emitting
element;
a light sensitive impedance element series-connected to said
photoelectromotive diode array, said light sensitive impedance
element changing its impedance value according to the intensity of
said optical signal from said light emitting element, wherein said
light sensitive impedance element comes into a small impedance
state when said optical signal from said light emitting element is
strong and it comes into a large impedance state when said optical
signal from said light emitting element is weak;
a zener diode parallel-connected with said light sensitive
impedance element;
at least one output MOSFET whose gate and source are connected to
respective ends of the series-connected circuit comprised of said
photoelectromotive diode array and said light sensitive impedance
element; and
a normally-ON drive transistor whose control electrode is connected
to the node of said photoelectromotive diode array and said light
sensitive impedance element, and whose one pair of conducting
electrodes are connected to the gate and the source of said output
MOSFET respectively.
3. A photo-coupler apparatus comprising:
a light emitting element for emitting light by an input signal;
a photoelectromotive diode array for generating photoelectromotive
force by receiving an optical signal from said light emitting
element;
a light sensitive impedance element series-connected to said
photoelectromotive diode array, said light sensitive impedance
element changing its impedance value according to the intensity of
said optical signal from said light emitting element;
a normal diode array parallel-connected with said light sensitive
impedance element;
at least one output MOSFET whose gate and source are connected to
respective ends of the series-connected circuit comprised of said
photoelectromotive diode array and said light sensitive impedance
element; and
a normally-ON drive transistor whose control electrode is connected
to the node of said photoelectromotive diode array and said light
sensitive impedance element, and whose one pair of conducting
electrodes are connected to the gate and the source of said output
MOSFET respectively.
4. The photo-coupler apparatus according to claim 3, wherein said
light sensitive impedance element comes into a small impedance
state when said optical signal from said light emitting element is
strong while it comes into a large impedance state when said
optical signal from said light emitting element is weak.
5. The photo-coupler apparatus according to claim 1, 2, or 3,
wherein said normally-ON drive transistor is a P-channel type or a
N-channel type Junction FET.
6. The photo-coupler apparatus according to claim 1, 2, or 3,
wherein said light sensitive impedance element, photoelectromotive
diode array, output MOSFET, and drive transistor are integrated
into one chip.
7. The photo-coupler apparatus according to claim 1, 2, or 3,
wherein said light sensitive impedance element stays in a small
resistance state for a certain time once said optical signal from
said light emitting element has been interrupted.
8. The photo-coupler apparatus according to claim 1, 2, or 3,
wherein another output MOSFET is further connected to said output
MOSFET in the form of anti-series connection with source common so
as to control an AC signal.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to a photo-coupler apparatus, and
more particularly, to a photo-coupler apparatus having at least one
MOSFET as an output contact.
2. Description of the Prior Arts
FIG. 1 shows the circuit diagram of a prior art photo-coupler
apparatus (first prior art). This prior art has been published in
Japanese unexamined patent publication TOKUKAISHO 57-107633 which
corresponds to U.S. Pat. No. 4,390,790, and EPS 0048146.
The photo-coupler apparatus of the first prior art has a light
emitting diode 1 at the primary side. The secondary side of this
device is comprised of the following: a first photoelectromotive
diode array 2a photo-coupled with light emitting diode 1; a second
photoelectromotive diode array 2b photo-coupled with light emitting
diode 1; a resistor 3 parallel-connected with second diode array
2b; a normally-ON drive FET 5; and an output MOSFET 4. In this
figure, numbers 8 and 8' show the input terminals and numbers 9 and
9' show the output terminals of this photo-coupler apparatus.
As shown in FIG. 1, the photo-coupler apparatus has two
photoelectromotive diode arrays 2a and 2b. Once the second
photoelectromotive diode array 2b has received a light from light
emitting diode 1, it creates an electromotive force. Because of
this electromotive force, drive FET 5, which is normally 0N, turns
off. Then, the gate-source capacitor of output MOSFET 4 is rapidly
charged by the photo-current from first photoelectromotive diode
array 2a.
Once the incident light from light emitting diode 1 is interrupted,
the accumulated charges between the gate and source of drive FET 5
are discharged through resistor 3, thus permitting drive FET 5 to
turn on. At this time, the charges accumulated between the gate and
source of output MOSFET 4 are rapidly discharged via the source and
drain of drive FET 5 which is now in an ON state, allowing MOSFET 4
to turn off quickly.
As described above, the photo-coupler apparatus of the first prior
art should include second photoelectromotive diode array 2b so as
to control drive FET 5. Due to the existence of array 2b, the chip
area of this apparatus increases, thus also increasing its
manufacturing cost.
In FIG. 2A, the circuit structure of a photo-coupler apparatus is
shown according to the second prior art of this invention. This
apparatus has been published in Japanese unexamined patent
publication TOKUKAISHO 63-99616 which corresponds to U.S. Pat. No.
4,873,202.
The second prior art photo-coupler apparatus has a light emitting
diode 1 at the primary side. The secondary side of this apparatus
is comprised of the following: a photoelectromotive diode array 2
which is photo-coupled with light emitting diode 1; an impedance
element 6 connected in series with array 2; normally-ON drive FET
5; and output MOSFET 4 connected to output terminals 9 and 9'.
In this photo-coupler apparatus, once photoelectromotive diode
array 2 receives a light from light emitting diode 1, a current
flows through impedance element 6. Due to this current, a voltage
difference, which can activate FET 5, arises between the source and
gate of FET 5. Therefore, this apparatus does not need second
photoelectromotive diode array 2b shown in FIG. 1 for the
activation of drive FET 5.
In this case, however, impedance element 6 limits the charging
current for the capacitor of MOSFET 4 when the resistance value of
element 6 is large. This fact makes the charging period for MOSFET
4 longer. Thus, the time T-on, which is the period from the signal
input to the turn-on of output MOSFET 4, becomes longer.
Impedance element 6 also works as a discharge resistor for
discharging the accumulated charges at the source and gate of FET
S. Therefore, if the resistance value of impedance element 6 is
large, the discharging period of FET 5 becomes longer. As a result,
time T-off, which is the period from the cut off of an input signal
to the turn-off of output MOSFET 4, becomes longer.
On the contrary, if the resistance value of impedance element 6 is
small, the current flowing through impedance element 6 must be
increased to obtain a sufficient voltage for the activation of
drive FET S. Therefore, the magnitude of minimum input current
I-ft, which is required to turn on MOSFET 4, becomes larger, thus
deteriorating the dynamic sensitivity of this device. As is evident
from the above mentioned explanation, there is a trade-off between
the switching times (T-on and T-off) and the minimum input current
I-ft of MOSFET 4. This trade-off prevents the complete improvement
of the characteristics of this photo-coupler apparatus.
FIG. 2B shows the characteristic curves of the second prior art
photo-coupler apparatus. The detail of this figure will be
explained later in conjunction with one embodiment of this
invention.
In FIG. 3A, the circuit structure of a photo-coupler apparatus is
shown according to the third prior art of this invention. This
apparatus has been published in Japanese unexamined parent
publication TOKUKAISHO 63-153916 which corresponds to U.S. Pat. No.
4,801,822.
This apparatus has a light emitting diode 1 at the primary side.
The secondary side of this apparatus is comprised of the following:
a photoelectromotive diode array 2 which is photo-coupled with
light emitting diode 1; impedance element 6' including a resistor
6a and a zener diode 6b connected in parallel with each other;
normally-0N drive FET 5; and output MOSFET 4 connected to output
terminals 9 and 9'.
In this photo-coupler apparatus of the third prior art, impedance
element 6' is comprised of the parallel circuit of resistor 6a and
zener diode 6b as mentioned above. Because zener diode 6b works as
a bypass for resistor 6a, most of the photo-current from array 2
flows through zener diode 6b, not through resistor 6a, when the
current amount is large. As a result, resistor 6b does not limit
the amount of charging current for MOSFET 4. This fact permits
resistor 6b to have a greater value of resistance without making
T-on longer. Time T-on and minimum input current I-ft can,
therefore, be improved simultaneously in the third prior art.
In said case, however, there are still some problems. That is, an
extra component, zener diode 6b, is necessary to construct the
photo-coupler apparatus. And, if resistor 6b has a large value of
resistance so as to reduce the amount of input current I-ft (that
is, to improve the dynamic sensitivity), time T-off becomes
longer.
FIG. 3B shows the characteristic curves of the third prior art
photo-coupler apparatus. The detail of this figure will be
explained later in conjunction with another embodiment of the
present invention.
To summarize the above mentioned results, the photo-coupler
apparatus of the prior arts have the following disadvantages:
(1) the photo-coupler apparatus having two photoelectromotive diode
arrays (in the first prior art) requires an additional chip area
for the installation of the second photoelectromotive diode array
in order to activate a drive FET, thus increasing the chip area as
well as the manufacturing cost;
(2) although it does not require the second photoelectromotive
diode array, the second prior art photo-coupler apparatus, in which
an impedance element is series-connected with a photoelectromotive
diode array, presents a trade-off relation between the switching
times (T-on and T-off) and the minimum current I-ft (that is,
sensitivity), thus preventing the improvement of the entire
characteristics of this apparatus;
(3) the third prior art photo-coupler apparatus, in which an
impedance element is comprised of a resistor and a zener diode
parallel-connected each other, can improve both T-on and I-ft
simultaneously, but it requires an extra component, zener diode 6b,
and time T-off becomes longer if a large value of resistor is used
to improve I-ft.
SUMMARY OF THE INVENTION
This invention has been made to overcome the above mentioned
disadvantages of the prior art photo-coupler apparatus.
Therefore, the objective of the present invention is to provide a
photo-coupler apparatus having at least one output MOSFET as an
output contact, the apparatus which is capable of shortening the
switching times of output contacts and improving its dynamic
sensitivity simultaneously.
In order to implement the above mentioned objective, the secondary
side of the photo-coupler apparatus of this invention is comprised
of a photoelectromotive diode array 2, a light sensitive impedance
element 10, at least one output MOSFET 4 connected to output
terminals 9 and 9', and a normally-ON drive transistor 5, as shown
in FIGS. 4, 7, and 10. Impedance element 10 is connected in series
with photoelectromotive diode array 2 and changes its impedance
value according to the intensity change of an input light. In
actuality, impedance element 10 has a small impedance value when
the light intensity from a light emitting element is strong, and a
large impedance value when the light intensity is weak.
According to the above mentioned structure, normally-ON drive
transistor 5, which is used to shorten the switching times of
output MOSFET 4, is driven by the voltage difference across element
10. When the light intensity is strong and so the charging current
for the gate-source capacitor of MOSFET 4 is large in amount,
impedance element 10 comes into a small impedance state, thus not
limiting the charging current for MOSFET 4. As a result, time T-on
of output MOSFET 4 is shortened.
On the other hand, when the light intensity is weak, impedance
element 10 comes into a large impedance state, thus generating a
voltage difference, which is sufficiently high to activate drive
transistor 5, across impedance element 10. Due to this fact, the
minimum current I-ft required to turn on output MOSFET 4 becomes
smaller, thus improving the dynamic sensitivity of this
apparatus.
In order to drive the photo-coupler apparatus in a high speed
switching mode, both time T-on and time T-off, which is the period
from the interruption of an input signal to the turn off of output
MOSFET, should be shortened. To this end, a large signal should be
applied to light emitting element 1 so as to emit a strong light.
When photoelectromotive diode array 2 receives a strong light, a
large amount of current is generated in this array and it charges
the gate-source capacitor of MOSFET 4 quickly. As a result, time
T-on is shortened.
On the other hand, time T-off can be shortened by adjusting the
light responding rate of impedance element 10. In other words, if
impedance element 10 is so arranged that it keeps the small
resistance state for a while (a period during which the charges
between the source and gate of transistor 5 can be discharged)
after an input signal having been interrupted, that is, no light is
emitted by light emitting element 1, drive transistor 5 rapidly
comes into an ON state during this period, thus shortening time
T-off.
As is evident from the above mentioned explanation, the
photo-coupler apparatus of this invention is capable of improving
its dynamic sensitivity and shortening the switching times of
output contacts simultaneously.
In addition, in order to further raise the limiting value of
charging up current for the MOSFET, a zener diode 10b may be
parallel-connected with light sensitive impedance element 10 as
shown in FIG. 8. In this structure, in the same manner as that of
the above mentioned photo-coupler apparatus, light sensitive
impedance element 10 has a large resistance when a light signal is
so weak that a high sensitivity is required, thus improving its
dynamic sensitivity. On the other hand, when the light signal is so
strong that a sufficient amount of current flows for charging
MOSFET 4, impedance element 10 comes into a small resistance state
and does not limit the charging up current. Thus, time T-on is
shortened. As a result, the photo-coupler apparatus having this
structure can improve the dynamic sensitivity and shorten time
T-on. Impedance element 10 may be fabricated to keep the small
resistance state for a while after an input light has been
interrupted. In this case, drive transistor 5 quickly turns on
during this period. Thus, time T-off is also shortened.
Still in addition, normal diode array 10c may be connected in place
of zener diode 6b as shown in FIG. 9. In this structure, the clamp
voltage of array 10c can be arranged by changing the number of
diodes. As a result, the photo-coupler apparatus having this
structure can further improve the charging efficiency.
These and other objectives, features, and advantages of the present
invention will be more apparent from the following detailed
description of preferred embodiments in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the circuit structure of a photo-coupler apparatus
according to the first prior art of this invention;
FIG. 2A shows the circuit structure of a photo-coupler apparatus
according to the second prior art of this invention;
FIG. 2B shows the V-I characteristics of some elements used in the
photo-coupler apparatus shown in FIG. 2A;
FIG. 3A shows the circuit structure of a photo-coupler apparatus
according to the third prior art of this invention;
FIG. 3B shows the V-I characteristics of some elements used in the
photo-coupler apparatus shown in FIG. 3A;
FIG. 4 shows the circuit structure of a photo-coupler apparatus
according to the first embodiment of this invention;
FIG. 5 is a cross sectional view of the impedance element shown in
FIG. 4;
FIG. 6A is a view showing the voltage variation of an input signal
for the photo-coupler apparatus shown in FIG. 4;
FIG. 6B is a view showing the resistance variation of the impedance
element shown in FIG. 4;
FIG. 6C is a view showing the output voltage variation from the
MOSFET shown in FIG. 4;
FIG. 7 shows the circuit structure of a photo-coupler apparatus
according to the second embodiment of this invention;
FIG. 8 shows the circuit structure of a photo-coupler apparatus
according to the third embodiment of this invention;
FIG. 9 shows the circuit structure of a photo-coupler apparatus
according to the fourth embodiment of this invention; and
FIG. 10 shows the circuit structure of a photo-coupler apparatus
according to the fifth embodiment of this invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 4 shows the circuit structure of a photo-coupler apparatus
according to the first embodiment of the present invention.
The photo-coupler apparatus of this embodiment has a light emitting
element 1 in the primary side. Element 1 emits a light after having
been activated by an input signal introduced through input
terminals 8 and 8'. The secondary side of this apparatus is
comprised of the following: photoelectromotive diode array 2 which
generates an electromotive force by receiving a light signal from
light emitting element 1; a light sensitive impedance element 10
whose impedance value varies according to the strength of the light
signal from light emitting element 1; an output MOSFET 4 whose gate
and source are connected with the respective ends of the series
connected circuit of photoelectromotive diode array 2 and impedance
element 10; and a normally-ON drive transistor 5. The control
electrode (base) of this transistor 5 is connected to the node of
photoelectromotive diode array 2 and impedance element 10. Also,
the conducting electrodes (source and drain) of this transistor 5
are connected with the gate and the source of MOSFET 4
respectively. The output of this apparatus is obtained through
output terminals 9 and 9'.
Light sensitive impedance element 10 presents a small impedance
value when a strong light is given by light emitting element 1. On
the contrary, it presents a large impedance value when a weak light
is given by light emitting element 1. In this embodiment, an
n-channel type junction FET (J-FET) is used as drive transistor 5.
FIG. 5 illustrates the cross-sectional structure of light sensitive
impedance element 10. This element is comprised of a diffused
resistor 54 formed in an island which is electrically isolated from
silicon substrate 51 by oxide film (SiO2) 57 and poll-silicon 58.
Boron (B) or antimony (Sb) may be used as the impurity which is
diffused into resistor 54. In this figure, 55 shows an oxide film
(SiO2) and 56 shows an aluminum wiring. In such a device, an
incident light is received by a part of diffused resistor 54 where
it is not covered by aluminum wirings 56. The impedance of this
element 10 can be changed by changing the shape of diffused
resistor 54 and the size of its exposed area.
A commercially available CdS light sensor may be used as impedance
element 10. However, it is appropriate that impedance element 10 is
integrated into one chip together with photoelectromotive diode
array 2 and FETs. In this case, the above mentioned diffused
resistor may be used as impedance element 10.
When an incident light from light emitting device 1 is strong,
light sensitive impedance element 10 has a small resistance. Due to
this strong light, however, a large current flows through element
10, thus inducing a voltage, which is sufficiently high for the
drive of transistor 5, across element 10. Transistor 5 then turns
off. In this case, therefore, most of the photo-current generated
by photoelectromotive diode array 2 is used in order to charge the
gate-source capacitor of MOSFET 4. Because its resistance value is
small, element 10, which is series-connected on the charging path,
does not limit the charging current for MOSFET 4. This fact permits
MOSFET 4 to rapidly turn on.
Once the incident light from light emitting element 1 is
interrupted, the accumulated charges between the gate-source
capacitor of transistor 5 are promptly discharged because the
resistance of element 10 stays small for a while. As a result,
transistor 5 rapidly turns on, allowing the gate and source of
MOSFET 4 to conduct. Then, the accumulated charges between the gate
and source capacitor of MOSFET 4 are promptly discharged,
permitting MOSFET 4 to come into an off-state quickly.
When the incident light from light emitting element 1 is weak,
element 10 has a large resistance. Due to this fact, a voltage,
which is sufficiently high for the drive of transistor 5, is
generated across element 10, in spite that photo-diode array 2
generates only a small amount of photo-current. Then, transistor 5
turns off so as to charge up the gate and source capacitor of
MOSFET 4 with the photo-current. MOSFET 4 is, thus, set to an
ON-state. Accordingly, even in the case where the current of an
input signal is small, and so, light emitting element 1 emits only
a weak light, MOSFET 4 can be set to an on-state. Thus, the
photo-coupler apparatus presents a high sensitivity.
In order to explain the operation of this photo-coupler apparatus,
the V-I characteristics of the second prior art apparatus will be
described first with referring to FIG. 2B. As mentioned before, the
photo-coupler apparatus of the second prior art has a simple
resistor as impedance element 6. FIG. 2B shows the operational
characteristics of this apparatus. Lines Ra, Rb and Rc in FIG. 2B
show the V-I characteristics of element 6 when it has a large,
small, or medium value of resistance. The MOSFET's operational
points, at which MOSFET 4 turns on to be a steady state, are shown
as the intersecting points of each V-I characteristic line and the
Vgs-Ids characteristic line of transistor 5. Therefore, current Ia,
Ib, or Ic of each operating point shows the minimum current
required to turn on MOSFET 4 in each case. In order to improve the
dynamic sensitivity of this apparatus, the minimum current should
be as small as possible.
On the other hand, when the photo-current is sufficiently large,
the amount of charging current for the gate-source capacitor of
MOSFET 4 is limited by the resistance value of impedance element 6.
Voc in FIG. 2B shows the open end voltage of photoelectromotive
diode array 2. For resistance values Ra and Rc, the maximum
currents which can flow through array 2 and impedance element 6 are
ia and ic. As is evident from this fact, when the resistance of
impedance element 6 is large, very little current flows in the
circuit. As the current becomes smaller, the period required for
the charging of MOSFET 4 becomes longer. Accordingly, a large
current is required for shortening time T-on.
To sum up, when impedance element 6 has a large resistance (Ra),
the operating current is very small, that is, Ia. This means the
dynamic sensitivity of this apparatus is very high. In this case,
however, T-on becomes longer due to the small current value, i.e.,
Ia. On the contrary, when element 6 has a small resistance value
(Rb), the limiting value of current becomes sufficiently large to
shorten T-on. However, the operating current becomes Ib, thus
deteriorating the dynamic sensitivity. In practice, the resistance
of element 6 is determined by finding a point of compromise around
the mean value, i.e., Ia in FIG. 2B.
As explained above, the resistance of impedance element 6 is
actually fixed in the second prior art. In said first embodiment of
the present invention, the resistance of impedance element 10
varies according to the intensity change of an input light. In
other words, when the intensity of light signal is weak, that means
a high sensitivity is required, the resistance is sufficiently
large (Ra), resulting in the high sensitivity. On the other hand,
when the intensity of light signal is strong to generate a large
current, the resistance of element 10 becomes smaller (Rb), thus
raising the limit value of current. This makes T-on shortened. As a
result, the apparatus of this embodiment can implement the high
sensitivity and the shortening of T-on simultaneously without
adding extra components such as zener diode 6a of the third prior
art.
In addition, if impedance element 10 stays in a small resistance
state for a certain period (a period during which the accumulated
charges between source and gate of transistor 5 are discharged)
after the input signal has been interrupted, transistor 5 rapidly
turns on to shorten T-off. FIGS. 6 show the relation between the
input signal for light emitting element 1 and the corresponding
output from the output terminals of this apparatus. Particularly,
FIG. 6A shows the voltage variation of the input signal, FIG. 6B
shows the resistance variation of impedance element 10 according to
the input voltage variation shown in FIG. 6A, and FIG. 6C shows the
output voltage variation of this apparatus according to the
resistance variation shown in FIG. 6B. As is evident from these
figures, the resistance of element 10 gradually increases during
time t2 to t3. The accumulated charges between the gate and source
of transistor 5 are promptly discharged during this time. As a
result, T-off is shortened.
To control the light responding rate of impedance element 10, there
are some approaches widely known. One of them is, for example, to
control the life timer of photo-carriers by irradiating electron
beams or proton beams and implanting ions into the resistor.
Another is to change the area size of the resistor so as to change
the parasitic capacitance of the resistor.
Next, the second embodiment of this invention will be explained
below with referring to FIG. 7. The photo-coupler apparatus shown
in FIG. 7 uses a p-channel junction FET 5a instead of n-channel
junction FET 5 used in the first embodiment of this invention.
Therefore, the operation and advantages of this embodiment are
almost the same as those of the first embodiment.
FIG. 8 shows the structure of a photo-coupler apparatus according
to the third embodiment of this invention. In addition to the
structure of the first embodiment, this photo-coupler apparatus has
an additional zener diode 10b which is parallel-connected with
light sensitive impedance element 10. Zener diode 10b in this
structure conducts when an input light is strong, thus removing the
limitation of charging up current for MOSFET 4 in cooperation with
light sensitive impedance element 10.
The operational characteristics of this embodiment will be
explained with referring to the characteristics of the third prior
art apparatus in which impedance element 6 is comprised of resistor
6a and zener diode 6b. In fact, FIG. 3B shows V-I characteristics
of transistor 5, zener diode 6b and resistor 6a shown in FIG.
3A.
As is evident from FIG. 3B, when the charging up current for the
gate-source capacitor of MOSFET 4 exceeds a certain value, it is
bypassed through zener diode 6b. Therefore, even if resistor 6a has
a large resistance value, the charging up current for MOSFET 4 is
not limited. As a result, the apparatus shown in FIG. 3A can
simultaneously implement the high dynamic sensitivity and the
shortening of time T-on. In spite of these advantages, the
apparatus shown in FIG. 3A is disadvantageous in that it requires
an extra component, e.i., zener diode 6b. In addition, if resistor
6a has a large value, time T-off becomes longer. This is because
resistor 6a is also used as the discharging resistor for transistor
5.
The resistance value of impedance element 6a is fixed as mentioned
before in the third prior art. On the contrary, the photo-coupler
apparatus of the third embodiment of this invention changes its
resistance value according to the intensity change of an incident
light. In the case where the incident light is so weak that a high
sensitivity is required, impedance element 10 has a large
resistance value (Ra), thus implementing the high sensitivity. On
the other hand, when the incident light is strong so that a large
amount of charging current is applied to MOSFET 4, impedance
element 10 represents a small resistance value (Rb shown in FIG.
2B) to remove the current limitation, thus shortening time T-on. As
a result, this apparatus can simultaneously implement the high
dynamic sensitivity and the shortening of time T-on. In addition,
after the incident light has been interrupted, impedance element 10
stays in a small resistance state for a certain period. This makes
transistor 5 turn on rapidly, thus also shortening time T-off.
FIG. 9 shows the circuit structure of a photo-coupler apparatus
according to the fourth embodiment of this invention. This
apparatus has normal diode array 10c instead of zener diode 10b
shown in the third embodiment.
In said third embodiment, to lower the pinch-off voltage of drive
transistor 5 is not difficult. But, it is very difficult to lower
the zener voltage of diode 10b below a certain value. Thus, when
the charging up current for MOSFET 4 is large, the voltage drop
(clamp voltage) caused by zener diode 10b becomes so large that the
voltage applied on the gate of MOSFET 4 becomes lower. This results
in the deterioration of the charging efficiency for MOSFET 4.
The fourth embodiment overcomes the above mentioned problem by
applying normal diode array 10c into the circuit. The clamp voltage
of diode array 10c can be arranged by changing the number of diodes
used in the array. Thus, by optimizing both the clamp voltage and
Vp on Vgs-Ids characteristic of transistor 5, the charging
efficiency of this apparatus can be further improved.
FIG. 10 shows the circuit structure of a photo-coupler apparatus
according to the fifth embodiment of this invention. The apparatus
of this embodiment has MOSFET 4' in addition to the structure of
the first embodiment. In fact, two MOSFETs 4 and 4' are
antiseries-connected each other with source common. Due to this
structure, the apparatus of this embodiment can control an AC
signal.
As explained above, the photo-coupler apparatus of this invention
has a structure to drive a normally-ON drive transistor, which is
provided in order to shorten the switching times of an output
MOSFET, by the voltage difference generated across a light
sensitive impedance element. When the incident light from a light
emitting element is strong, and so, the charging up current for the
gate-source capacitor of the MOSFET is large, the light sensitive
impedance element comes into a small resistance state. Accordingly,
the charging up current for the MOSFET is not limited by the
impedance element, thus shortening the switching-on period of the
MOSFET. On the other hand, when an incident light from the light
emitting element is weak, the impedance element presents a large
resistance value. Then, a considerable voltage difference to
turn-off the normally ON transistor arises across the light
sensitive impedance element. Due to this fact, the minimum current
required for turning on the output MOSFET is further reduced in
this invention. The dynamic sensitivity of this apparatus is, thus,
improved. In addition, the switching off time of the output MOSFET
can be shortened by adjusting the light responding rate of the
impedance element. As a result, a photo-coupler apparatus having an
improved dynamic sensitivity and shorter switching times of output
contacts can be obtained in this invention.
In addition, a zener diode may be parallel-connected with the light
sensitive impedance element so as to bypass the charging up current
for the output MOSFET. In this structure, zener diode conducts when
an input light is strong, thus removing the current limitation for
charging up the MOSFET in cooperation with the light sensitive
impedance element. Further, when the intensity of an input light is
weak, the impedance element presents a large resistance value, thus
improving the dynamic sensitivity of this apparatus. On the other
hand, when the intensity of the light signal is strong and so the
charging up current is large, the impedance element presents a
small resistance value. This enables to higher the limiting value
of charging current. Consequently, the apparatus of this invention
can implement the high dynamic sensitivity as well as the
shortening of the turn-on time. Further, the impedance element
stays in the small resistance state for a while after the input
signal has been interrupted. During this period, the drive
transistor comes into an off-state rapidly, thus also shortening
the period to turn off the MOSFET. So, a photo-coupler apparatus
having an improved dynamic sensitivity as well as shorter switching
times of output contacts can be obtained in this invention.
Still in addition, a normal diode array can be used in place of the
above mentioned zener diode. Due to this structure, the clamp
voltage of the diode array can be adjusted by changing the number
of diodes. Thus, a photo-coupler apparatus having an improved
charging efficiency can be obtained in this invention.
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